The present invention relates to molding compositions based on high viscosity, high-molecular weight polyalkylene terephthalates.
Polyethylene terephthalate having a viscosity number of &gt;75 cm.sup.3 /g can be manufactured and processed in a relatively convenient manner. The same holds true for polybutylene terephthalate with a viscosity number of &gt;120 cm.sup.3 /g. Polyesters having a higher viscosity are required for a variety of industrial uses. However, processing such polyesters necessitates a marked increase in manufacturing costs.
It is known from German Pat. Nos. 3,033,468 and 3,033,469 to produce higher-molecular polyalkylene terephthalates by solid phase recondensation from products having a lower molecular weight. However, manufacture is time- and energy-consuming, and the processability of molding compositions made therefrom is not improved.
It is also known that an increase in molecular weight of polyesters can be obtained during processing by crosslinking reactions with difunctional compounds, such as, for example, difunctional isocyanates. Thus, European Pat. No. 56 293 describes a process for premixing polyethylene terephthalate, or polymer mixtures based on polyethylene terephthalate, with organic diisocyanates and processing the mixture in a twin-screw extruder with a degasifying zone heated to a temperature of between 280.degree. and 320.degree. C. However, molding compositions containing aromatic diisocyanates prepared according to the teaching of that patent tend toward discoloration. Aliphatic or cycloaliphatic diisocyanates, which do not cause discoloring, are significantly less reactive and markedly more toxic, as compared with the aromatic diisocyanates.